If under the Set Gear Title you have a statement like requires Leatherworking 350, you can only get the set bonus for that gear IF you are a 350 Leatherworker. You can still wear the gear, just no set bonus.

Example: Felstalker is a three piece set that has a set bonus of +20 to HIT. A Hunter can wear all three pieces but no +20 Hit will be applied if they are not a LW'er.